Part 1: Top-level Architecture

In this and the following chapters, we will build a sample Akka.NET application
to introduce you to the language of actors and how solutions can be formulated
with them. It is a common hurdle for beginners to translate their project into
actors even though they don't understand what they do on the high-level. We will
build the core logic of a small application and this will serve as a guide for
common patterns that will help to kickstart Akka.NET projects.

The application we aim to write will be a simplified IoT system where devices,
installed at the home of users, can report temperature data from sensors. Users
will be able to query the current state of these sensors. To keep things simple,
we will not actually expose the application via HTTP or any other external API,
we will, instead, concentrate only on the core logic. However, we will write
tests for the pieces of the application to get comfortable and proficient with
testing actors early on.

Our Goals for the IoT System

We will build a simple IoT application with the bare essentials to demonstrate
designing an Akka.NET-based system. The application will consist of two main
components:

Device data collection: This component has the responsibility to maintain
a local representation of the otherwise remote devices. The devices will be
organized into device groups, grouping together sensors belonging to a home.

User dashboards: This component has the responsibility to periodically
collect data from the devices for a logged in user and present the results as
a report.

For simplicity, we will only collect temperature data for the devices, but in a
real application our local representations for a remote device, which we will
model as an actor, would have many more responsibilities. Among others; reading
the configuration of the device, changing the configuration, checking if the
devices are unresponsive, etc. We leave these complexities for now as they can
be easily added as an exercise.

We will also not address the means by which the remote devices communicate with
the local representations (actors). Instead, we just build an actor based API
that such a network protocol could use. We will use tests for our API everywhere
though.

The architecture of the application will look like this:

Top Level Architecture

When writing prose, the hardest part is usually to write the first couple of
sentences. There is a similar feeling when trying to build an Akka.NET system:
What should be the first actor? Where should it live? What should it do?
Fortunately, unlike with prose, there are established best practices that can
guide us through these initial steps.

When one creates an actor in Akka.NET it always belongs to a certain parent.
This means that actors are always organized into a tree. In general, creating an
actor can only happen from inside another actor. This 'creator' actor becomes
the parent of the newly created child actor. You might ask then, who is the
parent of the first actor you create? To create a top-level actor one must
first initialise an actor system, let's refer to this as the object System.
This is followed by a call to System.ActorOf() which returns a reference to
the newly created actor. This does not create a "freestanding" actor though,
instead, it injects the corresponding actor as a child into an already existing
tree:

As you see, creating actors from the "top" injects those actors under the path
/user/, so for example creating an actor named myActor will end up having
the path /user/myActor. In fact, there are three already existing actors in
the system:

/ the so-called root guardian. This is the parent of all actors in the
system, and the last one to stop when the system itself is terminated.

/user the guardian. This is the parent actor for all user created
actors. The name user should not confuse you, it has nothing to do with
the logged in user, nor user handling in general. This name really means
userspace as this is the place where actors that do not access Akka.NET
internals live, i.e. all the actors created by users of the Akka.NET library.
Every actor you will create will have the constant path /user/ prepended to
it.

/system the system guardian.

The names of these built-in actors contain guardian because these are
supervising every actor living as a child of them, i.e. under their path. We
will explain supervision in more detail, all you need to know now is that every
unhandled failure from actors bubbles up to their parent that, in turn, can
decide how to handle this failure. These predefined actors are guardians in the
sense that they are the final lines of defence, where all unhandled failures
from user, or system, actors end up.

Does the root guardian (the root path /) have a parent? As it turns out, it
has. This special entity is called the "Bubble-Walker". This special entity is
invisible for the user and only has uses internally.

Structure of an IActorRef and Paths of Actors

The easiest way to see this in action is to simply print IActorRef instances.
In this small experiment, we print the reference of the first actor we create
and then we create a child of this actor, and print its reference. We have
already created actors with System.ActorOf(), which creates an actor under
/user directly. We call this kind of actors top level, even though in
practice they are not on the top of the hierarchy, only on the top of the user
defined hierarchy. Since in practice we usually concern ourselves about actors
under /user this is still a convenient terminology, and we will stick to it.

Creating a non-top-level actor is possible from any actor, by invoking
Context.ActorOf() which has the exact same signature as its top-level
counterpart. This is how it looks like in practice:

First, we notice that all of the paths start with akka://testSystem/. Since
all actor references are valid URLs, there is a protocol field needed, which is
akka:// in the case of actors. Then, just like on the World Wide Web, the
system is identified. In our case, this is testSystem, but could be any other
name (if remote communication between multiple systems is enabled this name is
the hostname of the system so other systems can find it on the network). Our two
actors, as we have discussed before, live under user, and form a hierarchy:

akka://testSystem/user/first-actor is the first actor we created, which
lives directly under the user guardian, /user

akka://testSystem/user/first-actor/second-actor is the second actor we
created, using Context.ActorOf. As we see it lives directly under the first
actor.

The last part of the actor reference, like #1053618476 is a unique identifier
of the actor living under the path. This is usually not something the user needs
to be concerned with, and we leave the discussion of this field for later.

Hierarchy and Lifecycle of Actors

We have so far seen that actors are organized into a strict hierarchy. This
hierarchy consists of a predefined upper layer of three actors (the root
guardian, the user guardian, and the system guardian), thereafter the user
created top-level actors (those directly living under /user) and the children
of those. We now understand what the hierarchy looks like, but there are some
nagging unanswered questions: Why do we need this hierarchy? What is it used
for?

The first use of the hierarchy is to manage the lifecycle of actors. Actors pop
into existence when created, then later, at user requests, they are stopped.
Whenever an actor is stopped, all of its children are recursively stopped too.
This is a very useful property and greatly simplifies cleaning up resources and
avoiding resource leaks (like open sockets files, etc.). In fact, one of the
overlooked difficulties when dealing with low-level multi-threaded code is the
lifecycle management of various concurrent resources.

Stopping an actor can be done by calling Context.Stop(actorRef). It is
considered a bad practice to stop arbitrary actors this way. The recommended
pattern is to call Context.Stop(self) inside an actor to stop itself, usually
as a response to some user defined stop message or when the actor is done with
its job.

The actor API exposes many lifecycle hooks that the actor implementation can
override. The most commonly used are PreStart() and PostStop().

PreStart() is invoked after the actor has started but before it processes
its first message.

PostStop() is invoked just before the actor stops. No messages are
processed after this point.

We see that when we stopped actor first it recursively stopped actor second
and thereafter it stopped itself. This ordering is strict, allPostStop()
hooks of the children are called before the PostStop() hook of the parent is
called.

The family of these lifecycle hooks is rich, and we recommend reading the actor
lifecycle section of the reference for
all details.

Hierarchy and Failure Handling (Supervision)

Parents and children are not only connected by their lifecycles. Whenever an
actor fails (throws an exception or an unhandled exception bubbles out from
receive) it is temporarily suspended. The failure information is propagated to
the parent, which decides how to handle the exception caused by the child actor.
The default supervisor strategy is to stop and restart the child. If you don't
change the default strategy all failures result in a restart. We won't change
the default strategy in this simple experiment:

We see that after failure the actor is stopped and immediately started. We also
see a log entry reporting the exception that was handled, in this case, our test
exception. In this example we use PreStart() and PostStop() hooks which are
the default to be called after and before restarts, so we cannot distinguish
from inside the actor if it was started for the first time or restarted. This is
usually the right thing to do, the purpose of the restart is to set the actor in
a known-good state, which usually means a clean starting stage. What actually
happens though is that the PreRestart() and PostRestart() methods are called
which, if not overridden, by default delegate to PostStop() and PreStart()
respectively. You can experiment with overriding these additional methods and
see how the output changes.

The First Actor

Actors are organized into a strict tree, where the lifecycle of every child is
tied to the parent and where parents are responsible for deciding the fate of
failed children. At first, it might not be evident how to map our problem to
such a tree, but in practice, this is easier than it looks. All we need to do is
to rewrite our architecture diagram that contained nested boxes into a tree:

In simple terms, every component manages the lifecycle of the subcomponents. No
subcomponent can outlive the parent component. This is exactly how the actor
hierarchy works. Furthermore, it is desirable that a component handles the
failure of its subcomponents. Together, these two desirable properties lead to
the conclusion that the "contained-in" relationship of components should be
mapped to the "children-of" relationship of actors.

The remaining question is how to map the top-level components to actors. It
might be tempting to create the actors representing the main components as
top-level actors. We instead, recommend creating an explicit component that
represents the whole application. In other words, we will have a single
top-level actor in our actor system and have the main components as children of
this actor.

The first actor happens to be rather simple now, as we have not implemented any
of the components yet. What is new is that we have dropped using
Console.WriteLine() and instead use ILoggingAdapter which allows us to use
the logging facility built into Akka.NET directly. Furthermore, we are using a
recommended creational pattern for actors; define a static Props() method in
the the actor: